Primary carbonate and marl layers and limestone nodular horizons were intersected in OGCP Boreholes 1A, 2A, 3A, 3B, drilled into the depocentre of Palaeolake Olduvai. The various carbonate types were analysed, employing petrographic (including cathodo-luminescence), stable isotope, and sequence stratigraphic techniques, and recorded important information concerning lake evolution.

Primary carbonate and marl layers are preserved at the top of lake deepening cycles (lake-parasequences), marking maximum flooding, followed by lake withdrawal, and then fluvial erosion, leading to the next depositional cycle. Presence of pelagites and a meromict lake favouring anoxic conditions, show it to be not a playa lake, but a deeper fault-bound lake, akin to present day Lake Eyasi. Carbonate and marl layers were exclusively deposited when claystone facies show a geochemical magnesium anomaly associated with a phase of basaltic volcanism affecting the basin, marked by mafic tuffs and basaltic lava flows. Calcium was partitioned into evaporites, such as gypsum/anhydrite, together with the nodular horizons and carbonate soil profiles in the sequence. The resulting high Mg2+/Ca2+ ratio in the lake waters promoted dolomite deposition and replacement, and the formation of aragonite.

The nodular horizons yield rainfall isotope values and mark times when the lake was empty and the lake-bed fell under meteoric conditions, precipitating limestone nodules just below the sediment surface under pedogenic vadose, groundwater interface, and groundwater phreatic conditions. The nodular limestone horizons sit below erosional/hiatal surfaces and show vadose micritic and fibrous types to phreatic sparry and other accretionary type textures. Enterolithic to chickenwire textured nodules indicate the pseudomorphing of anhydrite nodules that form beneath salt marshes.

Thus, the two carbonate types, primary layers and nodular horizons, provided depth gauges, respectively for the extremes of lake expansion and emptying/drying out. In the case of nodular horizons, the maturity of the carbonate profile gives an indication of the magnitude of the hiatal time gap represented, ~ 1 kyr to 2 kyr for a single horizon to ~ 3 kyr to 9 kyr for more mature soil profiles. Emptying/drying out episodes of Palaeolake Olduvai were not uncommon, and often short-lived. A spectrum of hiatal disconformities in the Olduvai Basin vary from multimillennial hiatuses to mega-disconformities of ~ 40 kyr and ~ 75 kyr.